The present invention relates generally to synchronization methods and more particularly to an improved method and arrangement for deriving a reference signal from a pilot frequency utilizing phase-locked loop techniques. This arrangement is able to derive a common reference frequency from one of several preestablished pilot frequencies which are normally transmitted along with information signals on a microwave RF communications system.
Today's microwave communications systems are able to make efficient use of the crowded radio spectrum by combining a group of channels containing information signals onto a very high RF carrier utilizing multiplexing techniques. Such multiplex techniques depend on a synchronization signal which is transmitted along with this group of channels, and which is received and decoded for demultiplexing these groups of channels. This multiplexing process generally depends on a pilot frequency, such as 60, 64, 308, 512, or 564 KHz, being transmitted with the group of multiplexed channels over a microwave system. While such systems may require one or more pilot frequencies to be utilized, equipment design constraints necessarily dictate a common reference signal be derived from each of these possible pilot frequencies. Thus, the synchronization method and apparatus at the receivers in the microwave system is expected to acquire and maintain an internal reference frequency having high stability. Furthermore, the synchronization method and apparatus utilized to derive this reference frequency should provide an easily programmable method to select one of the possible pilot frequencies to be utilized on the system without having to remove and replace expensive crystal filters or other delicate hardware components.
Certain techniques are known which have provided synchronization capability to lock to a given pilot frequency and derive a reference signal. Such techniques generally utilize a voltage controlled crystal oscillator synchronized by a phase-lock loop to derive a reference frequency. These arrangements may also include additional circuit elements to provide a pilot frequency for providing master/slave capability.
One known method utilizes a voltage controlled crystal oscillator phase-lock loop to derive the reference signal. This phase-lock loop (PLL) is synchronized to a programmable synthesizer which reconstructs and locally generates the pilot frequency. This programmable synthesizer in turn is synchronized by the inputted pilot frequency which has first been filtered by a high selectivity crystal bandpass filter. While such an arrangement is suitable for some system designs, it nevertheless suffers from a serious drawback. This drawback is that the narrowband crystal filter must be specifically chosen for the particular pilot frequency being used. Thus, although the programmable synthesizer permits the circuit to be synchronized to a different pilot frequency while maintaining the same reference frequency output, a technician must specifically remove and replace the crystal bandpass filter to effect a change in pilot frequency.
A second known arrangement utilizes three overlapping and intertwined phase-locked loops to derive a common reference frequency from one of a group of possible pilot frequencies. This arrangement utilizes a voltage controlled crystal oscillator (VCXO) to drive a number of fixed dividers in order to derive the reference frequency. This VCXO with dividers drives a digital phase detector operating at the reference frequency and also develops the injection signal for down-converting the inputted pilot frequency to an intermediate frequency (IF). This IF is comparable in value to the common reference signal to be derived. While such an arrangement is suitable for some system designs, it nevertheless suffers from increased complexity and relatively slow phase-lock acquisition capability, due to the digital phase detector operating at the reference frequency. Thus each of these arrangements has serious limitations unique to the structure itself. Moreover, both of these methods and arrangements have the additional disadvantage of requiring the input level of the pilot frequency to fall within a narrow operating range.
Accordingly, there exists a need for an improved synchronization method and apparatus which is able to be easily programmed to select a given pilot frequency and derive a common reference signal. This method and arrangement should allow rapid lock acquisition capability while minimizing the complexity and number of elements used overall, as well as eliminating hardware changes.